1. Field of the Invention
The present invention relates to a color correcting method and apparatus for correcting a color shift of an object imaged by an image input device such as a digital camera or a CCD camera and for correcting a color shift in an image displayed on an image output device such as a sublimation printer in color DTP (DeskTop Publishing) and, more particularly, to a color correcting method and apparatus for performing color matching to correct the color of an imaged object or an output and displayed object to the color of the object under a daylight illuminant, using a spectral reflectance or transmittance for accurately expressing the physical properties (spectral colorimetry) of colors even if the object is photographed under any illuminant, and an image input device and an image output device (color reproduction device) having arbitrary characteristics are used.
2. Description of the Related Art
In observing an object with a human eye or imaging an object by an image input device such as a camera, the tint of the color of the object and the color of the object reproduced in a photograph are normally influenced by the difference in the type of illuminant (spectral distribution). The difference in the type of illuminant, i.e., the difference in the spectral distribution of an illuminant is called an illuminant change hereinafter.
Assume that an imaged object is observed as an image output at a sublimation printer or the like. When the illuminant in the photographing mode is different from that in the observation mode, the color of the photographed object has a color shift from the color of the observed output image due to the illuminant change between the photographing and observation modes. For example, a photograph taken under an incandescent or fluorescent lighting becomes reddish or greenish as compared with a photograph taken under a daylight illuminant. The reproduced image looks unnatural.
Even if a given output image is observed under different illuminants, a color shift may occur. The given output image observed under one illuminant may not be recognized as identical to the given output image observed under another illuminant.
A color shift may occur in accordance with the characteristics of an image input device for imaging an object or an image output device (or an image display device) for outputting the image.
In recent years, color DTP (DeskTop Publishing) has been very popular in personal publishing or intra-office printing. A digital camera or the like has been popularly used to allow direct editing and correction of a photographed digital image on a computer. In the color management system of general color DTP, proofreading is performed on the basis of a daylight illuminant such as a daylight illuminant D50. When the finished printed matter is observed with another illuminant, the color balance of the entire image is lost due to a color shift caused by the illuminant change. Therefore, the printed matter looks unnatural.
An image input device does not have any characteristics for correcting the color of an object under the incandescent or fluorescent lighting to the color of the object under the daylight illuminant. The color of the object observed with a human eye may often be different from that imaged by the image input device.
A White Point Mapping (WPM) method generally used in a video camera or the like and a color correcting method (M. J. Vrhel, H. J. Trussell: Color Correction Using Principal Components, Color research and application, Vol. 17, No. 5, pp. 328-338 (1992)) proposed by Vrhel et. al. using an estimated spectral reflectance are available as techniques for correcting color shifts caused by illuminant changes for the input images.
Since the WPM method is, however, a method of simply correcting only white to white under the daylight illuminant, the color balance of the image cannot be corrected. The color correcting accuracy in the color correcting method by Vrhel et. al. is insufficient in applications to apparatuses and cannot be used in color correction of a reproduced image having a color shift caused by the illuminant change.
A method of converting imaged colors into colors of an intermediate colorimetric system to perform color correction is available as a method of reproducing the colors of an imaged object without being affected by the characteristics of an image output device or the like. According to this method, however, the types of illuminants which allow the human eye to observe a given color as identical colors are limited (i.e., the number of spectral distributions of the illuminants is limited), and color matching is established in only a very limited environment (illumination). A variety of types of illuminations for observing colors output from an image output device, such as illuminations using an incandescent lighting, a fluorescent lighting, and a sunbeam, are available. It is impossible to always match the reproduced colors with the actual colors under these illuminants in the color reproduction method generally used in the state-of-the-art color DTP. No method is proposed to correct a reproduced image having a color shift caused by the illuminant change.